In recent years, methods for measuring three-dimensional (3D) ranges by which not only shape information on an object but also range information on the object can be acquired have been used in many fields. The methods are mainly classified into two methods: active measurement and passive measurement. In active measurement, an object is irradiated with light, an electric wave, a sonic wave, or the like, and obtained information is used. In passive measurement, an object is not irradiated with specific light, a specific electric wave, or the like that is used as an aid in measurement.
As passive measurement, stereo matching or the like can be used. As active measurement, a time of flight (TOF) method, photo cutting, or the like can be used. In stereo matching, different images are taken by a plurality of optical sensors, and range information is acquired according to the triangulation principle. In Patent Document 1, by performing stereo matching on a region that is smaller than a region common to different images, measurement time is shortened and measurement accuracy is increased.
In the TOF method, an object is irradiated with infrared light for a certain period, and reflected light is detected by an optical sensor in each pixel. Utilizing a difference between light irradiation start time and arrival time of reflected light at an optical sensor, range information is calculated from an irradiation period, a detection signal, and light speed. In Non-Patent Document 1, two detection signals are acquired by two infrared light irradiations. One of the advantages of the TOF method is the ease of downsizing of a semiconductor device.
In Patent Document 2, by alternately performing two-dimensional (2D) imaging and 3D imaging every other frame period, 2D information (the intensity, color, or the like of light reflected from an object) and 3D information (a distance from a light source to the object) are acquired using one pixel.